They can defend them, but cannot purport or maintain their clients as innocent. They can try to present mitigating factors or plea bargain to lesser charges (there was some talk that this is what was going to happen with the Rs). However, some defense attorneys, KNOWING their clients are guilty, will lie about it (which is why they would rather not know and won't ask their clients about their guilt). The R lawyers defended the Clintons in the Whitewater scandal. Plenty of lies went down there. Some defense attorneys have raised to to an art.